1. Field of the Invention
This invention relates to superheterodyne (down-converter) receivers having spurious responses through harmonic relationships and images interfering with an expected received echo or frequency, including apparatus and methods for calculating the interference frequencies.
2. Description of the Prior Art
Radar receivers employing a superheterodyne design have long been used to extract from space, usually by way of an antenna, electromagnetic frequency energy. In the radar technique, frequencies are radiated or emitted by a transmitter, usually by the same antenna which receives the echo. The superheterodyne design is used extensively also in other receivers, such as radios, televisions, communication links and the like. In the superheterodyne design, frequency energy is emitted from the transmitter. A narrow band width or window is provided at a precise time offset from the transmission time, in order to receive the expected echo or return of the transmitter frequency. The receiver thus can bracket the intended or expected echo or return frequency, eliminating as much noise or undesired frequencies as possible. Such devices frequently utilize a mixer-oscillator circuit which allows for the narrow banding of the frequency response of the receiver. Such a frequency conversion technique enables a designer to narrow the frequency response bandwidth of the receiver. Other or spurious responses are introduced, however, because of harmonic relationships and images. Frequently such spurious responses can be suppressed by auxiliary filtering or bandpassing.
Certain receiver applications are not susceptible to such suppression techniques, however. In such circumstances, it is desirable to eliminate or to "turn off" the emitter which might be the source of the spurious signal received by the receiver, which signal might be coincidental with the intended echo or received signal.
It has long been desired to identify the interfering emitted frequencies quickly and accurately so that the emitter emitting the interfering frequencies might be effectively neutralized. Various apparatus and devices have been developed in the past to correct this problem. In this regard, attention is directed to Boothby, U.S. Letters Pat. No. 2,546,147. Such superheterodyne receiving systems affording electronic assistance in distinguishing and determining the intended received frequency from spurious signals are useful. It is desired to have more simple, more economical and power or electronically independent means for distinguishing such spurious, emitted interference frequencies by manually operated, passive devices.